In some current communication networks, e.g. in a WCDMA (Wideband Code Division Multiple Access) system, the normal downlink (and uplink) modulation is QPSK (Quadrature Phase Shift Keying). A service HSDPA (High Speed Downlink Packet Access) introduces new modulation schemes. 16QAM (QAM=Quadrature Amplitude Modulation) is accepted modulation method in 3GGP standardization. A problem is how to generate this higher order constellation with existing (on hardware) QPSK modulators.
In WO 03/053021 QPSK modulators are used together or separate for constellation points of a higher order modulation. For example, as shown in FIG. 4, a spreader A QPSK modulator (SprA) handles constellation points of a 16QAM modulation which are marked by x-marks. A spreader B (SprB) handles constellation points of the 16QAM modulation which are marked by +-marks. The other constellation points are handled by using both modulators together. The weights of the QPSK spreaders can be constant for different 16QAM symbols, the spreader A having three times bigger weights (i.e. 9.5 dB) than spreader B. FIG. 4 illustrates a creation of the constellation points “0011” and “1101”. Discontinuous Transmission (DTX) bits are used for disabling branches of the QPSK modulators for forming the constellation points of the 16QAM modulation. According to the proposal disclosed in the WO 03/053021, a 16QAM is built up by using two spreaders with constant weights and variable DTX.